Detailed descriptions of communication networks and systems can be found in literature, such as in Technical Specifications published by, e.g., the 3rd Generation Partnership Project (3GPP). In such systems, user equipments (UE) can, e.g., access mobile services via an access network comprising a Radio Access Network (RAN) and a Core Network (CN). Examples of 3GPP-based communication networks include, for example, 2G GSM/GPRS (Global System for Mobile Communications/General Packet Radio Services), 3G UMTS (Universal Mobile Telecommunications System), and LTE (Long Term Evolution) EPS (Evolved Packet System). Examples of radio access networks (RAN) include GERAN (GSM/EDGE (Enhanced Data rates for GSM Evolution) RAN for 2G GSM/GPRS), UTRAN (Universal Terrestrial RAN for 3G UMTS), and E-UTRAN (Evolved UTRAN for LTE EPS). Examples of packet core networks include GPRS Core (for 2G and 3G) and Evolved Packet Core (for 2G, 3G UTRAN and E-UTRAN).
In communication systems, such as those mentioned above, a potential problem is that a number of UEs and/or other devices may simultaneously require communication network resources. This may induce a congestion or overload of network resources, which may potentially have serious consequences as the communication system may no longer be able to function properly or sufficiently well.
This problem of congestion or overload is becoming increasingly important in particular with the introduction in communication networks of Machine Type Communication (MTC), also sometimes referred to as Machine-to-Machine Communication (M2M), for example as currently specified by standardization bodies such as the 3GPP, see for example 3GPP TS 22.368 V.12.1.0 (2012-12). MTC is a form of data communication which involves one or more MTC devices that do not necessarily involve human interaction. MTC devices are an example of a UE. In other words, MTC devices constitute a subset of the term UE.
With the introduction of MTC in communication systems such as those described hereinabove, the potentially large number of MTC devices and the nature of MTC may pose challenges on the communication networks. For example, access control signaling resources become particularly exposed in some scenarios. For example, some scenarios include MTC devices in the form of sensor devices which monitor states of technological systems (e.g. industrial systems) or processes or sensor devices monitoring various environmental conditions such as temperature, pressure and vibrations. For such MTC devices (and the applications where these MTC devices are utilized) external events such as power grid failure, a pipeline damage, an earthquake or an industrial process failure may trigger a large amount of MTC devices wanting to access the communication network simultaneously, or substantially simultaneously, for the purpose of reporting the triggering events to their respective application servers. When a large number of MTC devices require network resources simultaneously, or substantially simultaneously, there is an increased risk of congestion or overload in the communication network. In addition to the above example scenario seen with respect to MTC devices, overload may of course also be caused by non-MTC devices, e.g. UEs that do involve human interaction. Such non-MTC devices may e.g. include devices such as cellphones, smartphones, tablet computers, gaming devices, personal digital assistants (PDAs), etcetera. To sum up, when a large number of UEs (e.g., MTC devices and/or non-MTC devices) require network resources simultaneously, or substantially simultaneously, there is an increased risk of congestion or overload in the communication network.
A known means for protecting the network access resources from overload in an LTE cell is known as the Access Class Barring (ACB) mechanism. For the purpose of this mechanism, each UE is a member of at least one Access Class (AC), which is stored in the USIM. An evolved NodeB (eNB) may announce the ACB state in each cell through the broadcast system information (SI). System Information Block Type 2 (SIB 2) of the SI lists the state of each AC through an Access Class Barring Factor (ACBF) associated with each AC, which has a value between 0 and 1. When a UE finds an AC in the SI which corresponds to one stored in the USIM, the UE generates a random value between 0 and 1. If the random value is lower than the ACBF of the concerned AC, the UE considers the cell as barred, i.e. it is not allowed to access it, for a random time period with a mean value governed by the Access Class Barring Time (ACBT) parameter included in the SI (in SIB 2). With the particular nature of MTC devices in mind the 3GPP is currently working on an extension of the ACB concept, called Extended Access Barring (EAB). EAB is a mechanism for the operator(s) to control mobile originating access attempts from UEs that are configured for EAB in order to prevent overload of the access network and/or the core network. In overload situations, the operator can restrict access from UEs configured for EAB while permitting access from other UEs.
Sometimes the ACB/EAB mechanism is not enough to protect a cell from overload. This may, for example, be because the mechanism may be slow to react to changes due to the pressure on the network access resources (because it relies on the rather infrequently transmitted system information), e.g. during sudden surges of access attempts from MTC devices which are more or less synchronized for one reason or the other (as exemplified above). It may also be because the chosen ACB/EAB parameters were not appropriate to handle the number of access attempts or because ACB/EAB was not used at all.
Another method for access load control is known from the US patent application publication US 2012/0163169 A1, which was published on 28 Jun. 2012. This publication describes an overload control apparatus and method for a MTC type communication service. The method described in this document resembles the EAB method in that it attempts to proactively notify MTC devices of an overload state in order to make them refrain from access attempts. A difference from the earlier-described EAB method is that the method and apparatus of this disclosure use a MAC (Medium Access Control) subheader instead of the system information to carry the overload indications to the MTC devices. More particularly, US 2012/0163169 A1 proposes to configure a so-called E/T/R/R/BI subheader of the MAC header to include an overload indicator. A concerned MTC device is required to check for possible overload indications before it attempts to access the network through the random access procedure. Hence, a MTC device which wants to access the network must first monitor the downlink until it receives a message including the MAC subheader triggered by a random access attempt from another UE, i.e. a UE different from the concerned UE. If the MAC subheader does not contain any overload indicator, the MTC device is allowed to initiate the random access procedure by transmitting a random access preamble to the base station. On the other hand, if the MAC subheader does include an overload indicator, the MTC device waits a certain time until it sends the random access preamble. If applying a procedure as disclosed in this disclosure, the base station may potentially send out a large amount of messages irrespective of the overload situation. Thus, unecessarily much system resources may potentially be consumed. Also, since MTC devices are supposed to check the overload situation before attempting to send their respective random access preambles, the overall procedure may become delayed in some situations. Potentially, the monitoring and reception of the MAC subheaders may also mean an increased energy consumption of the UE.